1. Field of the Invention
This invention relates to an image heating apparatus suitable if used as a fixing device in an image forming apparatus such as a copying machine or a printer using a recording method such as an electrophotographic method or an electrostatic recording method.
2. Description of Related Art
Heretofore, in a fixing apparatus provided in an image forming apparatus adopting an electrophotographic method, an electrostatic recording method or the like, use has been widely made of a so-called heat fixing apparatus for causing a recording material bearing an unfixed toner image thereon to pass through a nip portion formed between a fixing roller and a pressure roller rotated while being brought into pressure contact with each other to thereby fix the toner image as a permanent image on a recording material.
An example of a conventional heat fixing apparatus is shown in FIG. 9 of the accompanying drawings. The reference numeral 40 designates a fixing roller provided with heating means, and a halogen lamp 41 is disposed in a hollow mandrel 42 of aluminum having a thickness of the order of 1 mm to 4 mm so as to satisfy mechanical strength, and heating sufficient to fuse a toner on a recording material P from the interior of the hollow mandrel 42 is effected by the supply of electric power from a power source, not shown.
A substance generally having an absorption rate of 90% or greater for black (e.g. okitsumo or the like) is generally applied to the interior of the hollow mandrel 42 to make the absorption of radiant heat be the halogen lamp 41 good, and the inner surface of the hollow mandrel is made rough with a view to make the absorption rate high, and the roughness thereof Rz is 10 xcexcm or greater.
Also, in order that the toner on the recording material P may be fixed on the recording material P without being offset on the fixing roller, a mold releasable layer 43 of polytetrafluoroethylene (PTFE), parfluoroalchorytetrafluoroethylene copolymer (PFA) or the like exhibiting excellent performance in mold releasability is formed on the outer side of the hollow mandrel 42.
The mold releasable layer 43 is formed in a tube shape or formed by electrostatic spray, dipping application or the like on the hollow mandrel 42 having its outer surface subjected to blast process, etching process or the like and having its surface roughness Rz made into 5 xcexcm or greater, and obtains an adhesive force relative to the hollow mandrel 42.
Also, in some cases, in order to prevent offset caused by the surface of the fixing roller 40 being charged up by the conveyance of the recording material P, an electrically conductive material such as carbon black is mixed with the mold releasable layer 43.
Further, the hollow mandrel 42 of the fixing roller 40 is electrically earth-connected or grounded through a diode element, and has a bias applied thereto by bias applying means, not shown, to thereby prevent the surface of the fixing roller from being charged up and producing an offset image.
Also, a thermistor 44 is in contact with the surface of the fixing roller 40, and detects the temperature of the surface of the fixing roller to thereby on/off-control the supply of electric power to the halogen lamp 41 so as to heat the toner image on the recording material P at a moderate temperature.
On the other hand, the reference numeral 50 denotes a pressure roller brought into pressure contact with the fixing roller 40 in the lengthwisely opposite end portions thereof by pressure springs, not shown, to thereby nip and convey the recording material P. The pressure roller 50 comprises a mandrel 51 having applied to the outside thereof an elastic layer formed by molding silicon rubber or a sponge elastic layer 52 formed by foaming silicon rubber and further having applied to the outer layer thereof in a tube shape or as a coating a mold releasable layer 53 of PTFE, PFA, FEP or the like similar to that of the fixing roller 40.
Consequently, a fixing nip portion N of a sufficient nip width can be formed between the two rollers 40 and 50 by the elasticity of the pressure roller 50. The toner image on the recording material P nipped and conveyed by this fixing nip portion N can be fixed by the heating from the fixing roller 40.
Also, particularly a method whereby electric power is not supplied to a heat fixing apparatus during standby, and electric power consumption is minimized, and more particularly an example of a heat fixing method by a film heating process for fixing a toner image on a recording material through thin film between a heater portion and a pressure roller is proposed in Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 2-157878, Japanese Patent Application Laid-Open No. 4-44075, Japanese Patent Application Laid-Open No. 4-204980, etc.
FIG. 10 of the accompanying drawings schematically shows the construction of an example of a fixing apparatus of the film heating type. In FIG. 10, the fixing apparatus has a heating member (a heating body, hereinafter referred to as the heater) 61 fixedly supported by a stay holder (supporting member) 62, and an elastic pressure roller 50 brought into pressure contact with the heater 61 by pressure means with a fixing nip portion N of a predetermined nip width formed with heat-resistant thin film (hereinafter referred to as the fixing film) 63 interposed therebetween.
The heater 61 is heated and attempered to a predetermined temperature by electrical energization.
The fixing film 63 is a cylindrical or endless belt-shaped or rolled web-shaped member moved in the direction of arrow by the rotational force of driving means, not shown, or the pressure roller 50 while being in close contact with and sliding relative to the surface of the heater 61 in the fixing nip portion N.
When with the heater 61 heated and attempered to the predetermined temperature and the fixing film 63 moved in the direction of arrow, a recording material P bearing thereon an unfixed toner image as a material to be heated is introduced between the fixing film 63 and the pressure roller 50 in the fixing nip portion N, the recording material P is nipped and conveyed through the fixing nip portion N with the fixing film 63 while being in close contact with the surface of the fixing film 63. In this fixing nip portion N, the recording material and the toner image thereon are heated by the heater 61 through the fixing film 63, and the toner image on the recording material P is heat-fixed. That portion of the recording material which has passed through the fixing nip portion N is stripped from the surface of the fixing film 63 and is conveyed.
A ceramic heater is generally used as the heater 61 as a heating member. It is formed, for example, by forming an electrically energizing heat generating resistance layer of silver palladium (Ag/Pd), Ta2N or the like on the surface of a ceramic substrate of electrical insulativeness, good heat conductivity and low heat capacity such as alumina (that surface facing the fixing film 63) along the lengthwise direction of the substrate (a direction perpendicular to the plane of the drawing sheet of FIG. 10) by screen printing or the like, and covering the heat generating resistance layer forming surface with a thin glass protective layer. This ceramic heater 61 is such that the electrically energizing heat generating resistance layer thereof is electrically energized to thereby generate heat and the entire heater including the ceramic substrate and the glass protective layer rapidly rises in temperature. This temperature rise of the heater 61 is detected by temperature detecting means 64 installed on the back of the heater and is fed back to an electrical energization controlling portion, not shown. The electrical energization controlling portion controls the electrical energization of the electrical energizing heat generating resistance layer so that the temperature of the heater detected by the temperature detecting means 64 may be maintained at a predetermined substantially constant temperature (fixing temperature). That is, the heater 61 is heat and attempered to the predetermined fixing temperature.
The fixing film 63 has its thickness made as small as 20 to 70 xcexcm in order to efficiently give the heat of the heater 61 to the recording material P as the material to be heated in the fixing nip portion N. The fixing film 63 is comprised of three layers, i.e., a film base layer, an electrically conductive primer layer and a mold releasable layer, and the film base layer side is the heater side and the mold releasable layer is the pressure roller side. The film base layer is polyimide, polyamideimide, PEEK or the like which is high in insulativeness, and has heat resistance, and is formed to a flexible thickness of the order of 15 to 60 xcexcm. Also, the mechanical strength such as tear strength of the entire fixing film 63 is kept by the film base layer. The electrically conductive primer layer is formed by a thin layer having a thickness of the order of 2 to 6 xcexcm, and is electrically connected to the earth to prevent the charging-up of the entire fixing film. The mold releasable layer is a toner offset preventing layer for the fixing film 63, and is formed by coating fluorine resin such as PFA, PTFE or FEP which is good in mold releasability to a thickness of the order of 5 to 14 xcexcm. Also, like the fixing roller 40 of FIG. 9, in order to mitigate the charging-up of the surface of the fixing film 63 and prevent electrostatic offset, an electrically conductive material such as carbon black having specific resistance of the order of 103 xcexa9cm to 106 xcexa9cm is mixed with the mold releasable layer.
Also, the stay holder 62 is formed, for example, by a heat-resitant plastic member, and holds the heater 61 and serves also as a conveying guide for the fixing film 63. In order to enhance the slid ability relative to the fixing film 63, highly heat-resistant grease or the like is interposed between the fixing film 63 and the outer peripheral surface of the heater 61 or the stay holder 62. Also, the pressure member 50 is similar in construction to the pressure roller of the above-described heat fixing apparatus of the fixing roller type.
Also, in order to form the fixing nip portion N necessary for heat fixing between the fixing film 63 and the pressure roller 50, the opposite end portions of the stay holder 62 are pressurized against the pressure roller 50 side by pressure springs, not shown. Thereby, the heater 61 attached to the stay holder 62 is brought into close contact with the fixing film 63 over a portion of the circumferential direction and the entire lengthwise area of the pressure roller 50.
Also, the pressure roller 50 is rotatively driven and therewith, the fixing film 63 is driven to rotate by the surface of the pressure roller 50. In this state, the electrically energizing heat generating resistance layer formed on the heater 61 is electrically energized by a connector, not shown, through electrode portions formed on the opposite end portions of the heater 61. Thereby, the electrically energizing heat generating resistance layer is heated and rises in temperature to thereby heat and fix the toner image on the recording material nipped and conveyed by the fixing nip portion.
The above-described heat fixing apparatus, however, suffers from problems as mentioned below.
First, in the case of the heat fixing apparatus using the fixing roller 40, the thickness of the fixing roller mandrel 42 need to be of the order of 1 to 4 mm in order to satisfy the mechanical strength thereof, and has great heat capacity. Therefore, it is necessary to prelimarily heat the fixing roller 40 to a predetermined temperature before the image forming apparatus receives a print signal. This is because it is difficult to heat the fixing roller 40 from the room temperature to a temperature capable of fixing in the short time until the recording material P having an unfixed toner image formed thereon is conveyed to the heat fixing apparatus, and it becomes necessary to heat the fixing roller to a certain extent in the standby state before the image forming apparatus receives the print signal.
Therefore, when the power source of the image forming apparatus has been turned on from a state in which the fixing roller 40 has been cooled to the room temperature state, it has been necessary to heat the fixing roller 40 until the image forming apparatus becomes capable of receiving the print signal.
Also, it is necessary to heat the fixing roller 40 to a predetermined temperature by the electrical energization of the heater 41 during standby and therefore, energy was wastefully used.
Also, even when an attempt is made to cope with the problem by making the thickness of the mandrel 42 small, if an attempt is made to heat the fixing roller by the radiant of the heater 41 as in the above-described example of the related art, heat efficiency is not good and therefore, preliminary heating likewise becomes necessary when the recording material conveying speed becomes high by the higher speed of the image forming apparatus.
Also, when an attempt is made to make the temperature rising speed higher by making the thickness of the mandrel 42 smaller, the strength of the mandrel 42 is not sufficient and therefore, when the mandrel is pressurized with a strong pressure force, it is greatly flexed and comes to have cracks or the like therein, and this has led to a problem in durability.
On the other hand, in the heat fixing apparatus of the film heating type, the electrical energization of the heater 61 during standby as described above is not required, and even if the electrical energization of the heater 61 is effected after the image forming apparatus has received the print signal, it is possible to bring about a state in which heating is possible by the time when the recording material P arrives at the heat fixing apparatus. Consequently, from the viewpoint of energy saving, the heat fixing apparatus of the film heating type is an excellent heat fixing apparatus that does not waste energy.
However, the fixing film 63 is formed by a resin layer insufficient in heat conductivity, and has been unsuitable for the higher speed of the image forming apparatus. That is, when the image forming apparatus is made higher in speed, the heating speed given to the recording material P from the heater 61 through the fixing film 63 must be increased correspondingly to the higher speed of the apparatus, but for the fixing film 63 made of resin, there is a limitation even if there is taken a measure such as mixing a heat-conductive filler with the film, and it will become impossible to cope with still a higher speed.
So, as a fixing device which consumes little electric power and can cope with high-speed printing, it is proposed in Japanese Patent Application Laid-Open No. 5-61371, Japanese Patent Application Laid-Open No. 9-16004, etc. to make a rotary member contacting with the heater of metal.
By making the rotary member of metal, heat conductivity is improved and high-speed printing can be coped with.
However, there arise the new problems of the reduced durability and the rise of driving torque by the friction between the heater and the rotary member.
Also, when the surface of the metal is rough, the contact thermal resistance between the heater and the rotary member becomes great and the effect of making the rotary member of metal to thereby improve heat conduction decreases.
The present invention has been made in view of the above-noted problems and an object thereof is to provide an image heating apparatus that can cope with high-speed heating.
Another object of the present invention is to provide an image heating apparatus that is excellent in the heat conductivity until heat reaches a toner.
Still another object of the present invention is to provide an image heating apparatus in which the driving torque of a rotary member can be suppressed.
Yet still another object of the present invention is to provide an image heating apparatus that is excellent in durability.
A further object of the present invention is to provide an image heating apparatus comprising:
a heater; and
a rotary member rotated while contacting with the heater, the rotary member having a metal layer contacting with the heater;
wherein a surface roughness Rz of a surface of the metal layer which contacts with the heater is 3 xcexcm or less.
Still a further object of the present invention is to provide a rotary member for use in an image heating apparatus having:
a metal layer lying on the innermost surface of the rotary member;
wherein the surface roughness Rz of the inner peripheral surface of the metal layer is 3 xcexcm or less.
Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.